Lubricating oil composition

ABSTRACT

A lubricating oil composition which comprises a lubricant base oil and, incorporated therein, (A) from 0.01 to 0.2 mass % of a polyol compound having from 2 to 20 carbon atoms, (B) from 0.03 to 0.05 mass %, in terms of the amount of boron, of an alkali metal borate, (C) from 0.05 to 0.2 mass %, in terms of the amount of nitrogen, of an ashless dispersant, and (D) from 0.1 to 0.4 mass %, in terms of the amount of an alkaline earth metal, of an alkaline earth metal sulfonate having a total base number of 250 mg-KOH/g or more, the percentages being based on the total amount of the composition. The lubricating oil composition can simultaneously enhance the dynamic and the static friction coefficients in a wet clutch, and is suitable for use particularly in an automatic transmission.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lubricating oil composition, in particular, to a lubricating oil composition which is capable of combining the enhancement of the dynamic friction in a wet clutch and the enhancement of the static friction in the wet clutch, and is suitable for use particularly in an automatic transmission.

2. Description of the Prior Art

Nowadays, the improvement of the fuel efficiency of an automobile has become an urgent task for the purpose of reducing the amount of the carbon dioxide discharged, in the context of global environment problems. Accordingly, the contribution to the improvement of the fuel efficiency of an automobile is clearly required not only the improvement by the reduction of the friction in an engine, but also the improvement of an automatic transmission.

An automatic transmission is composed of a torque converter, a wet clutch, a gear-bearing mechanism and a hydraulic control mechanism for controlling these devices, and the transmission gear ratio thereof depends upon the numbers of teeth of gears engaging with each other. An automatic transmission has several types of gear mechanisms, and a desired transmission gear ratio is achieved by the appropriate selection of the gears providing the transmission gear ratio corresponding to a vehicle speed and a load. An automatic transmission has a number of wet clutches, and the appropriate selection of the gears providing the desired transmission gear ratio is, in turn, achieved by the appropriate combination of idling clutches and coupling clutches. At the time of the change of the transmission ratio, an operation is carried out in which coupling wet clutches are released and other wet clutches are coupled, and accordingly, a sufficiently high dynamic friction coefficient of a wet clutch is required for achieving a quick gear change. Further, since, for the purpose of transmitting an engine torque, coupling wet clutches must not slip until they are released, the wet clutch must exhibit a sufficiently high static friction coefficient. These friction coefficients are highly affected by the performance capability of a lubricating oil used as well as the material of a clutch.

In the design of an automatic transmission, the size and the number of wet clutches and the hydraulic pressure for pressing the clutch are decided corresponding to the out put of the engine combined therewith. A low friction coefficient of a wet clutch requires a large size of a clutch, a high number of clutches, an increased hydraulic pressure for pressing, or the like. A larger size of a clutch and a higher number of clutches result in a larger size of an automatic transmission itself comprising the clutches, and an increased hydraulic pressure for pressing leads to an excessive load upon the oil hydraulic pump incorporated in the automatic transmission. Accordingly, an enhanced friction coefficient in a wet clutch is useful for the miniaturization of an automatic transmission or the reduction in the pump loss, and thus an increasing need has arisen for the development of a lubricating oil capable of enhancing the friction coefficient in a wet clutch.

On the other hand, a continuously variable transmission uses no wet clutches for transmission, unlike an automatic transmission, but uses a wet clutch, for example, for switching between forward and backward movements. Further, most of continuously variable transmissions combined with a torque converter have an integrated lock-up clutch being effective for improving the gas mileage. Accordingly, an increasing need has arisen for the development of a lubricating oil capable of enhancing the friction coefficient in a wet clutch also in a continuously variable transmission, as in an automatic transmission.

Heretofore, a lubricating oil composition comprising a boron-containing ashless dispersant, an alkaline earth metal sulfonate, a phosphorus-based additive and the like in an optimized formulation has been disclosed as a lubricating oil composition capable of providing a high dynamic friction coefficient or a high static friction coefficient in a wet clutch (for example, see U.S. patent application Publication No. 2001/0034305 A1, corresponding to Japanese Patent Laid-Open Publication No. 2001-279286). Further, a lubricating oil composition comprising an oil-soluble metal salt being overbased with an alkaline earth metal borate and a compound having a long chain alkyl group and an amino group in the same molecule in an optimized formulation has been disclosed as a lubricating oil composition being capable of providing a high dynamic friction coefficient and static friction coefficient in a wet clutch and being reduced in a change with the passage of time (for example, see Japanese Patent Laid-Open Publication No. 5-148492). Still further, there has been disclosed a fluid for an automatic transmission and a wet clutch which contains an alcohol or a polyol having 12 or less carbon atoms in a molecule and 100 ppm or less of a metal-based detergent in terms of the metal, for the purpose of improving the dynamic friction coefficient and/or the static friction coefficient, and exhibits a dynamic friction coefficient of about 0.135 at the maximum (for example, see U.S. Pat. No. 5,817,605, corresponding to Japanese Patent Laid-Open Publication No. 5-186788). Still further, a lubricating oil composition comprising a diol compound having an alkyl or alkenyl group having 8 to 30 carbon atoms has been disclosed for improving friction characteristics such as a static friction coefficient (for example, see Japanese Patent Laid-Open Publication No. 2000-087061). However, the above-mentioned compositions has failed to achieve satisfactory dynamic and static friction coefficients, and accordingly the further improvement of friction coefficients is still requested.

SUMMARY OF THE INVENTION

The present invention is intended to respond to the above request, and the object of the present invention is to provide a lubricating oil composition exhibiting a further enhanced dynamic friction coefficient and static friction coefficient in a wet clutch.

The present inventors have intensively and extensively investigated for solving the above-mentioned task. As a result, it has been found that a lubricating oil composition comprising a specific polyol compound, an alkali metal borate, an ashless dispersant and a specific metal-based detergent can enhance a friction coefficient synergistically. The present invention has been completed, based on the above finding.

The present invention consists in a lubricating oil composition which comprises a lubricant base oil and, incorporated therein,

-   -   (A) 0.01 to 0.2 mass % of a polyol compound,     -   (B) 0.03 to 0.05 mass %, in terms of the amount of boron, of an         alkali metal borate,     -   (C) 0.05 to 0.2 mass %, in terms of the amount of nitrogen, of         an ashless dispersant, and     -   (D) 0.1 to 0.4 mass %, in terms of the amount of an alkaline         earth metal, of an alkaline earth metal sulfonate having a total         base number of 250 mg-KOH/g or more, the percentages being based         on the total amount of the composition.

The above polyol compound (A) is preferably an alcohol having three or more hydroxyl groups.

The above polyol compound (A) preferably has three to ten carbon atoms.

The above ashless dispersant (C) is preferably a mixture of non-modified succinimide and a boric acid modified succinimide.

The lubricating oil composition of the present invention is preferably used in an automatic transmission.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described below in more details.

As a lubricant base oil in the lubricating oil composition of the present invention, use may be made of an arbitrary mineral oil and/or synthetic oil being used as a base oil in a conventional lubricating oil.

Specifically, the above mineral oil may be a paraffinic oil, a naphthenic oil, a normal paraffin or the like which is produced by a method comprising subjecting a crude oil to atmospheric distillation and vacuum distillation to provide a lubricant fraction and then subjecting the lubricant fraction to a purification treatment comprising an appropriate combination of purification processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing and clay treatment.

The above synthetic oil is not particularly limited and may be a poly-α-olefin (a 1-octene oligomer, a 1-deecene oligomer or an ethylene-propylene oligomer, or the like) or a hydrogenation product thereof, an isobutene oligomer or a hydrogenation product thereof, an isoparaffin, an alkylbenzene, an alkylnaphthalene, a diester (di-tridecyl glutarate, di-2-ethylhexyl adipate, di-isodecyl adipate, di-tridecyl adipate, di-2-ethylhexyl sebacate or the like), a polyol ester (trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethyl hexanoate, pentaerythritol-2-ethyl pelargonate, or the like), a polyoxyalkylene glycol, a dialkyl diphenyl ether, polyphenyl ether or the like.

The above mineral oils may be used alone or in combination.

In a lubricant base oil in the lubricating oil composition of the present invention, one or more of a hydrorefined or hydrocracked mineral oil, an iso-paraffinic oil which is produced by the isomerization of a GTL (gas to liquid) wax from the Fisher-Tropsh process or the like, a wax being obtained in the step of dewaxing a lubricating oil and containing a large amount of normal paraffins, or the like, and a poly-α-olefin based synthetic oil such as 1-decene oligomer and/or a hydrogenation product thereof are preferably used in an amount of 50 mass % or more, more preferably in an amount of 70 mass % or more, since they can further enhance the activity of an additive and thus further enhance the dynamic friction coefficient and the static friction coefficient.

The dynamic viscosity of the above lubricant base oil is not particularly limited, but, at 100° C., the lower limit value of the dynamic viscosity thereof is generally 1 mm²/s, preferably 1.5 mm²/s, more preferably 2 mm²/s, particularly preferably 5 mm²/s, and the upper limit value thereof is generally 20 mm²/s, preferably 10 mm²/s, more preferably 8 mm²/s. A lubricant base oil exhibiting a dynamic viscosity at 100° C. of the above-mentioned lower limit value or higher allows the further enhancement of the dynamic viscosity and the static viscosity of the present lubricating oil composition and a base oil exhibiting a dynamic viscosity of the above-mentioned upper limit value or lower allows the reduction in the agitation resistance of the composition, which results in the formation of a composition being further excellent in gas-mileage performance capability.

The component (A) in the lubricating oil composition is a polyol compound, and specifically includes an alcohol having two or more hydroxyl groups represented by the following general formula (1): R¹(OH)_(a)   ( 1) wherein R¹ represents a hydrocarbon group and a represents an integer of 2 or more, and a condensed product of the alcohol.

The above-mentioned hydrocarbon group is not particularly limited in the carbon number thereof, and preferably has from 2 to 20 carbon atoms, more preferably 3 to 10 carbon atoms, and particularly preferably 3 to 6 carbon atoms, since such a hydrocarbon group can provide a lubricating oil exhibiting a further enhanced friction coefficient. The symbol a is an integer of 2 or more, as mentioned above. The symbol a is preferably an integer of 3 or more, since such an alcohol can provide a lubricating oil exhibiting a further enhanced friction coefficient, and is preferably an integer of 10 or less, more preferably an integer of 5 or less from the view point of the solubility into a lubricant base oil. The structure of the above hydrocarbon group is not particularly limited and may be a saturated hydrocarbon group (it may be straight or branched), an unsaturated hydrocarbon group (it may be straight or branched and the position of a double bond is not limited), an aromatic hydrocarbon group or a cyclic hydrocarbon group, wherein the aromatic hydrocarbon group and a cyclic hydrocarbon group may be substituted with a saturated hydrocarbon group (it may be straight or branched) or an unsaturated hydrocarbon group (it may be straight or branched and the position of a double bond is not limited) at an arbitrary position. The above hydrocarbon group is preferably a saturated hydrocarbon group or an unsaturated hydrocarbon group, particularly preferably a saturated hydrocarbon group. The position of the hydroxyl substituent in the above hydrocarbon group is not limited.

Specific examples of the component (A) in the present invention include diols such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butylene glycol, 1,4-butylene glycol, 1,2-pentane diol, 1,2-hexane diol, 1,2-heptane diol, 1,2-octane diol, 1,2-decane diol, 1,2-dodecane diol, 1,2-tetradecane diol, 1,2-hexadecane diol and 1,2-octadecane diol, glycerol, trimethylol ethane, trimethyol propane, erythritol, pentaerythritol, erythrite, arabitol, sorbitol, mannitol and the like, and condensed products thereof. Of these, glycerol, trimethyol propane, trimethylol ethane and pentaerythritol are preferred, and glycerol is particularly preferred.

The above-mentioned polyol compound (A) can be used alone or in combination.

The lower limit value of the content of the component (A) in the lubricating oil composition of the present invention is 0.01 mass %, preferably 0.02 mass %, particularly preferably 0.05 mass %, and the upper limit value thereof is 0.2 mass %, preferably 0.15 mass %, particularly preferably 0.1 mass %, all the percentages being relative to the total amount of the composition. When the content of component (A) is lower than the lower limit value, the resultant composition does not exhibit satisfactory effect on the enhancement of the dynamic friction coefficient. When the content of the component (A) exceeds the upper limit value, the solubility of the component into the composition is greatly reduced, which results in the undesirable lowering of the storage stability of the composition.

The component (B) of the lubricating oil composition of the present invention is an alkali metal borate.

The alkali metal borate of the component (B) is preferably used in the form of a hydrate. Specific examples thereof include lithium borate hydrate, sodium borate hydrate, potassium borate hydrate, rubidium borate hydrate and cesium borate hydrate. Sodium borate hydrate and potassium borate hydrate are particularly preferred as the component (B).

The alkali metal borate can be provided, for example, as a dispersion of fine particles of potassium borate hydrate or sodium borate hydrate which has been prepared by dissolving potassium hyroxide or sodium hydroxide and boric acid in water in a manner such that the resultant solution has an atomic ratio (boron/alkali metal) of boron to an alkali metal (potassium or sodium) of 2.0 to 4.5, adding the resultant solution to an oil solution containing a neutral alkaline earth metal sulfonate or a succinimide based ashless dispersant, subjecting the resultant mixture to vigorous agitation to form a water-in-oil type emulsion, and then subjecting the emulsion to dewatering.

The above alkali metal borate may be used alone or in combination.

The lower limit value of the content of the component (B) in the lubricating oil composition of the present invention is 0.03 mass %, preferably 0.035 mass %, and the upper limit value thereof is 0.05 mass %, preferably 0.045 mass %, all the percentages being in terms of the amount of boron and relative to the total amount of the composition. When the content of component (B) is lower than the lower limit value, the resultant composition does not exhibit satisfactory effect on the enhancement of dynamic friction coefficient. When the content of the component (B) exceeds the upper limit value, an undesirable lowering of the storage stability of the composition can take place.

The component (C) of the lubricating oil composition of the present invention is an ashless dispersant.

Examples of an ashless dispersant as the component (C) include a nitrogen-containing compound having, in a molecule thereof, at least one alkyl group or alkenyl group having from 40 to 400 carbon atoms and a derivative thereof, and a modification product of an alkenyl succinimide, and use can be made of one or more compounds arbitrarily selected from the above.

The above alkyl group or an alkenyl group may be linear or branched. Specific preferred examples thereof include a branched alkyl group and a branched alkenyl group derived from an oligomer of an olefin such as propylene, 1-butane, isobutylene or the like or a co-oligomer of ethylene and propylene.

The above alkyl group or alkenyl group has from 40 to 400 carbon atoms, preferably from 60 to 350 carbon atoms. When the alkyl group or alkenyl group has less than 40 carbon atoms, the solubility of the dispersant compound into a lubricant base oil is reduced to an undesirable level. When the alkyl group or alkenyl group has more than 400 carbon atoms, the flowability of the lubricating oil composition is reduced to an undesirable level.

Specific examples of the component (C) include one or more compounds selected from among:

-   -   (C-1) a succinimide having, in a molecule thereof, at least one         alkyl group or alkenyl group having from 40 to 400 carbon atoms         and a derivative thereof,     -   (C-2) a benzylamine having, in a molecule thereof, at least one         alkyl group or alkenyl group having from 40 to 400 carbon atoms         and a derivative thereof, and     -   (C-3) a polyamine having, in a molecule thereof, at least one         alkyl group or alkenyl group having from 40 to 400 carbon atoms         and a derivative thereof.

More specific examples of the succinimide according to the above (C-1) include the compounds represented by the general formula (2):

wherein R²represents an alkyl group or alkenyl group having from 40 to 400 carbon atoms, preferably from 60 to 350 carbon atoms, and b represents an integer of from 1 to 5, preferably from 2 to 4,

and the compounds represented by the general formula (3) :

wherein R ³ and R⁴ independently represent an alkyl group or alkenyl group having from 40 to 400 carbon atoms, preferably from 60 to 350 carbon atoms, and c represents an integer of from 0 to 4, preferably from 1 to 3.

The above succinimide involves the so-called mono-type succinimide which has the structure obtained by the addition of succin anhydride to one end of a polyamine through imidation and is represented by the general formula (2), and the so-called bis-type succinimide which has the structure obtained by the addition of succin anhydride to both ends of a polyamine and is represented by the general formula (3). In the production of the composition of the present invention, use can be made of both types of succinimide and a mixture thereof. The mono-type of succinimide is preferred for use in the composition of the present invention, since the mono-type can provide a lubricating oil composition exhibiting further enhanced friction coefficient.

More specific examples of the benzylamine according to the above (C-2) include the compounds represented by the general formula (4):

wherein R⁵ represents an alkyl group or alkenyl group having from 40 to 400 carbon atoms, preferably from 60 to 350 carbon atoms, and d represents an integer of from 1 to 5, preferably from 2 to 4.

The above benzylamine can be prepared, for example, by reacting a polyolefin (for example, a propylene oligomer, polybutene, an ethylene-α-olefin copolymer and the like) with phenol, to form an alkyl phenol, and then reacting the alkyl phenol with formaldehyde and a polyamine (for example, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, or the like) by the Mannich reaction.

More specific examples of the benzylamine according to the above (C-2) include the compounds represented by the general formula (5): R⁶—NH—(CH₂CH₂NH)_(e)—H   (5) wherein R⁶ represents an alkyl group or alkenyl group having from 40 to 400 carbon atoms, preferably from 60 to 350 carbon atoms, and e represents an integer of from 1 to 5, preferably from 2 to 4.

The method for preparing the above polyamine is not limited. The polyamine can be prepared, for example, by chlorinating a polyolefin such as a propylene oligomer, polybutene, an ethylene-α-olefin copolymer, and then reacting the chlorinated product with ammonia or a polyamine such as diethylene triamine, triethylene tetramine, tetraethylene pentamine or pentaethylene hexamine.

The nitrogen content of the above nitrogen compound is not limited. Generally, the nitrogen content is preferably from 0.01 to 10 mass %, more preferably from 0.1 to 10 mass %, from the view points of abrasion resistance, oxidation stability, friction characteristics and like.

The derivatives of the above-mentioned nitrogen compounds include the so-called acid-modified compound which is prepared by reacting the above-mentioned nitrogen compound with a monocarboxylic acid (a fatty acid or the like) having from 2 to 30 carbon atoms or a polycarboxylic acid having from 2 to 30 carbon atoms such as oxalic acid, phthalic acid, trimellitic acid or pyromellitic acid, to thereby neutrize or amidate a part or all of the residual amino groups and/or imino groups; the so-called boric acid modified compound which is prepared by reacting the above-mentioned nitrogen compound with boric acid, to thereby neutrize or amidate a part or all of the residual amino groups and/or imino groups; the so-called phosphoric acid modified compound which is prepared by reacting the above-mentioned nitrogen compound with phosphoric acid, to thereby neutrize or amidate a part or all of the residual amino groups and/or imino groups; a sulfur-modified compound which is prepared by reacting the above-mentioned nitrogen compound with a sulfur compound; a modified compound which is prepared by subjecting the above-mentioned nitrogen compound to a combination of two ore more selected from among the acid modification, the boric acid modification, the phosphoric acid modification and the sulfur modification; and the like.

The above ashless dispersant (C) may be used alone or in combination. It is preferred that the ashless dispersant comprises a boric acid modified succinimide since it provides a lubrication oil composition capable of further enhancing the dynamic friction coefficient in a wet clutch, and it is particularly preferred that the ashless dispersant comprises a mixture of a boric acid modified succinimide and unmodified succinimide since it provides a lubrication oil composition capable of further enhancing the static friction coefficient in a wet clutch. These succinimide and modified succinimides are preferably of a mono type from the same reason as mentioned above.

The lower limit value of the content of the component (C) in the lubricating oil composition of the present invention is 0.05 mass %, preferably 0.08 mass %, in terms of the amount of nitrogen, and the upper limit value thereof is 0.2 mass %, preferably 0.18 mass %, all the percentages being relative to the total amount of the composition. When the content of component (C) is lower than the lower limit value, the resultant composition does not exhibit satisfactory effect on the enhancement of the friction coefficient. When the content of the component (C) exceeds the upper limit value, the flowability of the component at a low temperature is markedly and undesirably reduced.

In the present invention, when a boric acid modified succinimide is used as the ashless dispersant (C), the ratio of boron to nitrogen (mass ratio) of the modified succinimide is preferably 0.5 or higher, more preferably 0.6 or higher, particularly preferably 0.8 or higher, and the ratio is preferably 2 or lower, more preferably 1 or lower.

Further, in the present invention, when a mixture of unmodified succinimide with a boric acid modified succinimide is used as the ashless dispersant (C), the ratio of boron to nitrogen (mass ratio) in the mixture is preferably 0.5 or higher, more preferably 0.55 or higher, and is preferably 2 or lower, more preferably 1 or lower, because a mixture having such a boron to nitrogen ratio provides a lubricating oil composition capable of further enhancing the dynamic and static friction coefficients in a wet clutch.

When use is made of the lubricating oil composition of the present invention containing a boric acid modified succinimide as mentioned above, the content of the boric acid modified succinimide is preferably 0.035 mass % or more, more preferably 0.04 mass percent or more, particularly preferably 0.045 mass % or more, and is preferably 0.2 mass % or less, more preferably 0.1 mass % or less, particularly preferably 0.08 mass % or less, in terms of the amount of boron, from the view point of the trade-off between the improvement of the dynamic friction coefficient in a wet clutch and the oxidation stability of the resultant composition.

The component (D) of the lubricating oil composition of the present invention is an alkaline earth metal sulfonate and the total base number of the alkali earth metal sulfonate is preferably 250 mg KOH/g or higher, from the view point of satisfactory enhancement of the dynamic and static friction coefficients in a wet clutch. The term “total base number” used herein denotes the total base number measured by the perchloric acid method in accordance with section 7 of JIS K2501 “Petroleum products and lubricants—Determination of neutralization number”.

More specific examples of alkaline earth metal sulfonates with the total base number of 250 mg KOH/g or higher include a basic salt which is produced by providing a neutral salt (a normal salt) prepared, for example, in such a way that an alkyl aromatic sulfonic acid obtained by the sulfonation of an alkyl aromatic compound having a molecular weight of, for example, from 100 to 1,500, preferably from 200 to 700, is reacted directly with an alkaline earth metal base such as an oxide or hydroxide of an alkaline earth metals such as magnesium and, calcium or a combination thereof, or such a sulfonic acid is once converted to an alkali metal salt such as a sodium salt or a potassium salt, and then the alkali metal salt is substituted with an alkaline earth salt, and adding an excessive alkaline earth metal salt and an alkaline earth metal base (a hydroxide or an oxide of an alkaline earth metal) to the neutral salt and heating the resultant mixture in the presence of water; and a perbasic salt (a super basic salt) which is produced by reacting the above neutral salt (a normal salt) with a base of an alkaline earth metal in the presence of carbonic acid gas and/or a boron compound such as boric acid or a borate. These reactions are generally carried out in a solvent (an aliphatic hydrocarbon solvent such as hexane, an aromatic hydrocarbon solvent such as xylene, a light lubricant base oil or the like).

Specific examples of an alkyl aromatic sulfonic acid used herein include the so-called petroleum sulfonic acids and synthetic sulfonic acids.

Generally, as the petroleum sulfonic acid, use is made of a sulfonation product of an alkyl aromatic compound component contained in the lubricant fraction of a mineral oil and the so-called mahogany acid being by-produced during the production of the white oil. As the synthetic sulfonic acids, use is made of a sulfonation product of an alkylbenzene having a linear or branched alkyl group which is by-produced in a plant for producing am alkylbenzene as a material for a cleaning agent or is produced by the alkylation of a polyolefin to benzene, or a sulfonation product of di-nonyl naphthalene. The sulfonating agent for sulfonating an alkyl aromatic compound is not particularly limited, and typically is fuming sulfuric acid or sulfuric acid.

Above-mentioned alkaline earth metal sulfonates are commercially available generally in the form of a fluid diluted with a light lubricant base oil or the like. Use is generally made of a material having metal content of from 1.0 to 20 mass %, preferably, of from 2.0 to 16 mass %.

The lower limit value of the content of one or more types of the component (D) in the lubricating oil composition of the present invention is 0.1 mass %, preferably 0.15 mass %, more preferably 0.2 mass %, more preferably 0.2 mass %, and the upper limit value thereof is 0.4 mass %, preferably 0.35, mass %, more preferably 0.3 mass %, in terms of the amount of an alkaline earth metal, all the percentages being relative to the total amount of the composition. When the content of component (D) is lower than the lower limit value, the resultant composition does not exhibit satisfactory effect on the enhancement of the friction coefficient. When the content of the component (D) exceeds the upper limit value, the oxidaton stability of the component is undesirably reduced.

In the lubricant composition of the present invention, use can be made of known additives for a lubricating oil, for example, an extreme-pressure agent or an abrasion preventing agent (phosphorus based, sulfur based, phosphorous-sulfur based, boron based, or the like) except the component (B), and a metallic detergent except the component (D), a viscosity index improver, a friction modifier, an antioxidant, a corrosion inhibitor, an antirust, a metal inactivation agent, an anti-foaming agent, etc.

In the present invention, in particular, at least one type of the additives is preferably incorporated in the composition which is selected from the group consisting of a viscosity index improver, a phosphorus based additive, a friction modifier, an oxidation inhibitor and an anti-foaming agent.

Specific examples of the viscosity index improvers which can be used together in the lubricating oil composition of the present invention include the so-called non-dispersion type viscosity index improvers such as copolymers from one or more monomers selected from various methacrylates, or hydrogenation products thereof, and the so-called dispersion type viscosity index improvers prepared by copolymerizing various methacrylates containing a nitrogen compound. Specific examples of other viscosity index improvers include non-dispersion or dispersion type ethylene-α-olefin copolymers (α-olefins include propylene, 1-butene and 1-pentene )and hydrogenation products thereof, polyisobutylene and hydrogenation products thereof, hydrogenated styrene-diene copolymers, styrene-maleic anhydride ester copolymers, and polyalkylstyrenes.

The molecular weight of these viscosity index improvers is preferably selected in view of shear stability. Specifically, it is desirable that the dispersion type and non-dispersion type polymethacrylates have a number average molecular weight of 5,000 to 150,000, preferably 5,000 to 35,000, that the polyisobutylene and hydrogenation products thereof have a number-average molecular weight of 800 to 5,000, preferably, 1,000 to 4,000, and that ethylene-α-olefin copolymers and hydrogenation products thereof have a number-average molecular weight of 800 to 150,000, preferably, 3,000 to 12,000.

In the present invention, a polymethacrylate, in particular, a dispersion type polymethacrylate is preferably used, since it can provide a lubricating oil composition which further enhances the friction coefficient in a wet clutch.

In the present invention, a lubricating oil composition may contain one or more compounds arbitrarily selected from these viscosity index improvers in an arbitrary amount. Generally, it is desirable that the composition contains a viscosity index improver in an amount of from 0.1 to 40.0 mass % relative to the amount of the composition.

Phosphorus-based additives which can be used in combination with the lubricant composition of the present invention include arbitrary compounds which are generally used as a phosphorus-based additive for a lubricating oil, and examples of such phosphorus based additives are monophosphate esters, diphosphate esters, triphosphate esters, monophosphite esters, diphosphite esters, triphosphite esters, and salts of these esters with amines or alkanol amines.

No particular limitation is imposed on the content of the above phosphorus-based additives. Generally, the content of the phosphorus based additive is preferably from 0.005 to 0.2 mass % of in terms of phosphorus based on the total amount of the composition. In the case of less than 0.005 mass % in terms of phosphorus, the resultant composition exhibits no effect on the improvement of the abrasion resistance, while in the case of more than 0.2 mass % in terms of phosphorus the oxidation stability is adversely affected.

Friction modifiers which can be used in combination with the lubricating oil composition of the present invention include arbitrary compounds which are generally used as a friction modifier for a lubricating oil. Examples of such friction modifiers include amine compounds, fatty acid esters, fatty acid amides, fatty acids and metal salts of a fatty acid which have, in a molecule thereof, at least one alkyl or alkenyl group having from 6 to 30 carbon atoms, in particular, at least one straight-chain alkyl or alkenyl group having from 6 to 30 carbon atoms.

Specific examples of the amine compounds include straight-chain or branched, preferably straight-chain aliphatic monoamines having from 6 to 30 carbon atoms, and straight-chain or branched, preferably straight-chain aliphatic polyamine, or alkylene oxide adducts of these aliphatic amines. Specific examples of the fatty acid esters include esters of straight-chain or branched, preferably straight-chain fatty acid having from 7 to 31 carbon atoms with aliphatic monohydric alcohols or aliphatic polyhydric alcohols. Specific examples of the fatty acid amides are amides of straight-chain or branched, preferably straight-chain fatty acids having 7 to 31 carbon atoms with aliphatic monoamines or aliphatic polyamines. Specific examples of fatty acids are straight-chain or branched, preferably straight-chain fatty acids having 7 to 31 carbon atoms. Specific examples of the metal salts of a fatty acid include alkaline earth metal salts (magnesium salts, calcium salts, and the like) and zinc salts, of straight-chain or branched, preferably straight-chain fatty acids having from 7 to 31 carbon atoms. It is particularly preferred that the lubricating oil composition of the present invention contains a fatty acid based and/or ester based friction modifier among the above-mentioned friction modifiers, since such a friction modifier can provide a composition which retains the dynamic friction coefficient and static friction coefficient at a high level and also is excellent in gear change characteristics (μ0/μd).

In the present invention, a lubricating oil composition may contain one or more compounds arbitrarily selected from these friction modifiers in an arbitrary amount. Generally, it is desirable that the composition contains a friction modifier in an amount of from 0.01 to 5.0 mass %, preferably from 0.03 to 3.0 mass %, relative to the amount of the composition.

An antioxydant which may be used in combination with the lubricant composition of the present invention can be any compound which is generally used as an antioxidant for a lubricating oil, for example, a phenolic or amine compound.

Specific examples of such oxidation inhibitors include alkylphenols such as 2-6-di-tert-butyl-4-methylphenol, bisphenols such as methylene-4,4-bisphnol (2,6-di-tert-butyl-4-methylphenol), naphthylamines such as phenyl-α-naphthylamine, dialkyldiphenylamines, zinc dialkyldithiophosphates such as zinc di-2-ethylhexyldithiophosphate, esters of (3,5-di-tert-butyl-4-hydroxyphenyl) fatty acid (propionic acid or the like) or (3-methyl-5-tert-butyl-4-hydroxyphenyl) fatty acid (propionic acid or the like) with a mono- or poly-hydric alcohol such as methanol, octanol, octadecanol, 1,6-hexanediol, neopentyl glycol, thiodiethylene glycol, triethylene glycol or pentaerythritol.

The lubricating oil composition of the present invention may contain one or more compounds arbitrarily selected from these compounds in an arbitrary amount.

Generally, it is desirable that the composition contains these compounds in an amount of from 0.01 to 5.0 mass %, relative to the total amount of the composition.

A defoaming agent which may be used in combination with the lubricant composition of the present invention can be any compound which is generally used as a defoaming agent for a lubricating oil, and such defoaming agents include silicones such as dimethylsilicone and fluorosilicone. The lubricating oil composition of the present invention may contain one or more compounds arbitrarily selected from these compounds in an arbitrary amount. Generally, it is desirable that the composition contains these compounds in an amount of from 0.001 to 0.05 mass %, relative to the total amount of the composition.

The invention will be further illustratively by way of the following examples and comparative examples. It is to be noted that the scope of the present invention should not be limited by the examples.

EXAMPLES Example 1 and Comparative examples 1 to 5

A lubricating oil composition according to the present invention (Example 1) and lubricating oil compositions for comparison (Comparative examples 1 to 5) were prepared through compounding various lubricant base oils and additives given in Table 1. The amounts of the respective additives were on the basis of the total amount of the lubricating oil composition.

The fatigue life of each of the resultant compositions was evaluated by a friction properties test for a wet clutch described in the following (1). The results of the evaluation for the respective compositions are together described in Table 1.

(1) The Friction Properties Test for a Wet Clutch

The friction properties test for a wet clutch was conducted in accordance with JASO M348-95 “Test method for friction property of automatic transmission fluids” using an SAE No. 2 friction tester. This test is consisted of a dynamic friction test and a static friction test. In the dynamic friction test, after a clutch was rotated at 3,600 rpm and with an inertial mass of 0.343 kg·m² under no load, the rotation of the clutch was stopped by the application of a pressure thereto. A friction coefficient was calculated from the torque occurring at a relative rotation of 1,800 rpm and the resulting value was defined as the dynamic friction coefficient. In the static friction test, a clutch was rotated at a relative rotation of 0.7 rpm with the application of a pressure to the clutch and a friction coefficient was calculated from the torque occurring thereupon. The friction coefficient at the maximum torque at which the clutch starts to slide with a low speed of rotation was defined as the static friction coefficient. In this test, each of the dynamic and static friction tests was conducted repeatedly and the capability to enhance the friction coefficient of each composition was evaluated with the friction coefficient after 3,000 cycles. In the above-mentioned test, a general paper-based wet friction material was used as a clutch material. The term “a paper based wet friction material” used herein denotes a composite material prepared by reinforcing a special paper comprising a fiber such as a natural cellulose pulp or a synthetic aramid pulp and a filler such as diatomaceous earth, cashew dust or graphite, with a phenol resin or the like. The above test was conducted with respect to commercially available high performance lubricating oils for an automatic transmission.

As a result, in the tests wherein the above commercial high performance lubricating oils were used in place of the lubricating oil composition of the present invention, the dynamic friction coefficients were approximately from 0.13 to 0.14, and the static friction coefficients were approximately 0.11 even at the highest values, and there was found no commercial high performance lubricating oils exhibiting a dynamic coefficient of 0.16 or higher and a static friction coefficient of 0.12 or higher. TABLE 1 comparative comparative comparative comparative comparative example 1 example 1 example 2 example 3 example 4 example 5 Composition Base oil Refined mineral oil ¹⁾ 86.498 86.548 87.098 89.498 88.498 82.998 mass % (A) Polyol compound Polyol compound A ²⁾ 0.05 0.05 0.05 0.05 0.05 (B) Alkaline earth Boron based extreme 0.6 0.6 0.6 0.6 0.6 metal borate pressure agent ³⁾ Boron content (mass %) 0.038 0.038 0.000 0.038 0.038 0.038 (C) Ashless Ashless dispersant A ⁴⁾ 2.0 2.0 2.0 2.0 2.0 2.0 dispersant Boron containing ashless 3.0 3.0 3.0 3.0 3.0 dispersant B ⁵⁾ Nitrogen content (mass %) 0.11 0.11 0.11 0.04 0.11 0.11 (D) Metal based Alkaline earth metal 2.0 2.0 2.0 2.0 detergent sulfonate A ⁶⁾ Alkaline earth metal 5.5 sulfonate B ⁷⁾ Alkaline earth metal 0.24 0.24 0.24 0.24 0.00 0.24 content (mass %) Other additives Viscosity index improver ⁸⁾ 5.0 5.0 5.0 5.0 5.0 5.0 Friction modifier A ⁹⁾ 0.05 0.05 0.05 0.05 0.05 0.05 Friction modifier B ¹⁰⁾ 0.2 0.2 0.2 0.2 0.2 0.2 Antioxidant A ¹¹⁾ 0.3 0.3 0.3 0.3 0.3 0.3 Antioxidant B ¹²⁾ 0.3 0.3 0.3 0.3 0.3 0.3 Defoaming agent ¹³⁾ 0.002 0.002 0.002 0.002 0.002 0.002 Dynamic friction coefficient SAE No. 2 tester 0.163 0.153 0.153 0.154 0.150 0.147 Static friction coefficient SAE No. 2 tester 0.130 0.132 0.131 0.072 0.090 0.067 ¹⁾ hydrogenated refined mineral oil (kinetic viscosity at 100° C.: 6.0 mm²/s, viscosity index: 125) ²⁾ glycerol ³⁾ potassium borate (boron content: 6.3 mass %) ⁴⁾ polybutenyl succinimide (mono type, nitrogen content: 2.2 mass %) ⁵⁾ boric acid modified polybutenyl succinimide (mono type, boron content: 2 mass %, nitrogen content 2.3 mass %) ⁶⁾ petroleum-based Ca sulfonate (total base number: 300 mg KOH/g, Ca content: 12.2 mass %) ⁷⁾ petroleum-based Ca sulfonate (total base number: 100 mg KOH/g, Ca content: 4.3 mass %) ⁸⁾ dispersion type polymethacrylate ⁹⁾ carboxylic acid based agent ¹⁰⁾ ester-based agent ¹¹⁾ dialkyldiphenylamine-based agent ¹²⁾ bisphenol-based agent ¹³⁾ polydimethylsiloxane

As apparent from the results given in Table 1, the lubricating oil composition according to the present invention (Example 1) exhibited excellent friction coefficients, that is, the dynamic friction coefficient of 0.16 or higher and the static friction coefficient of 0.12 or higher.

Whereas, (A) a lubricating oil composition which contains no polyol compounds (Comparative example 1), (B) a lubricating oil composition which contains no alkali metal borate (Comparative example 2), (C) a lubricating oil composition which contains an ashless dispersant in an amount less than that defined by the present invention (Comparative example 3) and (D) a lubricant composition which contains no specified alkaline earth sulfonate (Comparative examples 4 and 5) showed all a dynamic friction coefficient of approximately 0.15, which indicates that the dynamic friction coefficient is enhanced by a synergistic effect of components (A)-(D) in the composition of the example 1 of the present invention. Further, the static friction coefficients in the case of the lubricating oil compositions of Comparative examples 3-5 were all significantly lower.

EFFECT OF THE INVENTION

The lubricating oil composition, which has the above described constitution, is excellent in the effect of enhancing the dynamic friction coefficient and the static friction coefficient, is significantly effective for the miniaturization of a transmission for the transfer of power and the reduction of pump loss, and thus is expected to contribute to the improvement of the fuel efficiency in an automobile. 

1. A lubricating oil composition which comprises a lubricant base oil and, incorporated therein, (A) from 0.01 to 0.2 mass % of a polyol compound having from 2 to 20 carbon atoms, (B) from 0.03 to 0.05 mass %, in terms of the amount of boron, of an alkali metal borate, (C) from 0.05 to 0.2 mass %, in terms of the amount of nitrogen, of an ashless dispersant, and (D) from 0.1 to 0.4 mass %, in terms of the amount of an alkaline earth metal, of an alkaline earth metal sulfonate having a total base number of 250 mg-KOH/g or more, the percentages being based on the total amount of the composition.
 2. The lubricating oil composition according to claim 1, wherein the polyol compound (A) is an alcohol having three or more hydroxyl groups.
 3. The lubricating oil composition according to claim 1, wherein the polyol compound (A) has from 3 to 10 carbon atoms.
 4. The lubricating oil composition according to claim 1, wherein the ashless dispersant (C) is a mixture of non-modified succinimide and a boric acid modified succinimide.
 5. The lubricating oil composition according to claim 4, wherein it is a lubricating oil composition for use in an automatic transmission.
 6. The lubricating oil composition according to claim 2, wherein the polyol compound (A) has from 3 to 10 carbon atoms.
 7. The lubricating oil composition according to claim 2, wherein the ashless dispersant (C) is a mixture of non-modified succinimide and a boric acid modified succinimide.
 8. The lubricating oil composition according to claim 3, wherein the ashless dispersant (C) is a mixture of non-modified succinimide and a boric acid modified succinimide.
 9. The lubricating oil composition according to claim 2, wherein it is a lubricating oil composition for use in an automatic transmission.
 10. The lubricating oil composition according to claim 3, wherein it is a lubricating oil composition for use in an automatic transmission.
 11. The lubricating oil composition according to claim 4, wherein it is a lubricating oil composition for use in an automatic transmission.
 12. The lubricating oil composition according to claim 6, wherein the ashless dispersant (C) is a mixture of non-modified succinimide and a boric acid modified succinimide.
 13. The lubricating oil composition according to claim 12, wherein it is a lubricating oil composition for use in an automatic transmission. 